20m would be a good band for me to use, and it will just about fit, so
I was thinking of a 1/4 wavelength radiator with a 1/4 wavelength

You can easily make antennas that are hidden. For example, get some of that
aluminium tape they used on windows, then use it to make a loop on a large
window. The alarm tapes also have contact pads that you can use to make
contacts with them to feed the antenna. Such an antenna will work well for
transmitting and receiving. You can change the direction by opening the
window.

You could fasten the antenna to a packinmg crate lid, or even use the metal
band around the crate, as I did. Hang it on the wall and put a dartboard on
the other side. When finished just turn it over, or even leave it
permamently hidden behind the dartboard. Put a lamp over the board to excuse
the cable. http://web.telia.com/~u85920178/antennas/crate.htm

You could fix the aluminum tape to a carpet, then roll it up when finished.

Do you have the capability of adding a new plastic drainpipe? Looks good
from outside, but can hide an antenna inside ;-)

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If you are having trouble with neighbours, then make a wooden antenna using
brush-handles, or even a fishing pole. Let the neighbours see it and make a
fool of themselves when they accuse you of disturbance to the TV sets. Then
you can use the real antenna another day without fear. ;-)